This invention relates to a carrier-recovery circuit and particularly to a baseband carrier-recovery circuit for use in the demodulator of multiplex radio equipment utilizing four (4) levels of phase shift keying modulation.
The carrier recovery system utilizing the non-linear operation (logical treatment) in the baseband frequency is generally called the COSTAS system because a phase lock loop called a COSTAS loop is used. In the COSTAS system, signals from respective circuits are processed in the base-band frequency and therefore the system can be realized easier than other systems which processing the signals in a higher frequency range.
For example, one of the COSTAS systems will be explained, referring to an example of recovery of the reference carrier signal from the four (4) levels of a modulated PSK signal.
(i) The four (4)-level modulated PSK input signal is branched in two directions and is respectively mixed with the reference carrier signal having a phase difference of 90 degrees, for the purpose of in-phase quadrature detection. PA1 (ii) A first doubled signal (sin 2.theta.) is generated from the baseband signals (sin .theta., cos .theta.), having the phase difference of 90 degrees, sent from the in-phase quadrature detecting means. PA1 (iii) In a same way, the second doubled signal (cos 2), having a phase difference of 90 degrees from said first doubled signal, is generated from the baseband signals. PA1 (iv) A quadrupled signal (sin 40) is generated from the first and second doubled signals. PA1 (v) Since the phase modulated components which are applied to the four phases of the reference carrier signal can be eliminated by generating a quadrupled signal, the reference carrier signal can be obtained by using the quadrupled signal for oscillation control of a VCO (Voltage Controlled Oscillator) via the loop filter.
The process for generating the first doubled signal (sin 2.theta.) explained in item (ii) above, that is, the method of inputting respectively the two outputs (sin .theta., cos .theta.) of the quadrature detecting means into the exclusive-OR (EX-OR) gate circuit, is known. The equation (1) indicated below is used. ##EQU1##
In addition, the process for obtaining the second doubled signal (cos 2.theta.) having the phase difference of 90 degrees from the first doubled signal as indicated in item (iii) above, that is, the method for obtaining the square values of the outputs of the respective quadrature detecting means (or full-wave rectifying circuit) and thereafter extracting a difference between the second doubled signal and the first doubled signal, is also know. The equation (2) indicated below is used. EQU (sin .theta.).sup.2 -(cos .theta.).sup.2 =cos 2.theta. (2)
The succeeding process for obtaining the quadrupled signal (sin 4.theta.) is explained in item (iv) above, and the EX-OR of the first and second doubled signals are obtained as in item (ii) above.
As explained above, the method for obtaining the first doubled signal (sin 2.theta.) and the quadrupled signal (sin 40) can be realized easily with a simple structure using the EX-OR gate circuit, but a plurality of square circuits (or full-wave rectifying circuit) and a subtraction (or addition) circuit is needed in order to obtain the second doubled signal (cos 2.theta.), complicating the circuit structure.